The present invention relates two-dimensional image sensors for detecting electromagnetic radiation (X-rays, visible light, infrared light, etc.) images.
A conventional two-dimensional image sensor for electromagnetic radiation images is provided with semiconductor sensors arranged in a two-dimensional matrix form. Each semiconductor sensor is equipped with an electric switch and, as it detects X-rays, visible light, and other kinds of radiation (hereinafter, the description will focus on X-rays which represent all kinds of radiation), produces electric charge (electron-hole). The electric switches are activated one row at a time to measure the electric charge produced by the semiconductor sensor in each column.
Two-dimensional image sensors of this kind are described in terms of specific structure and detecting principles in, for example, D. L. Lee, et al., A New Digital Detector for Projection Radiography, SPIE, 2432, pp.237-249,1995 (published in May 1995); L. S. Jeromin, et al., Application of a-Si Active-Matrix Technology in a X-Ray Detector Panel, SID ""97 DIGEST, page 91-94, 1997 (published in May 1997); Japanese Laid-Open Patent Application No. 6-342098/1994 (Tokukaihei 6-342098; published on Dec. 13, 1994); and other documents.
The following will discuss the arrangement and detecting principles of a conventional two-dimensional image sensor in reference to FIG. 4 and FIG. 5. FIG. 4 is a plan view and a cross-sectional view taken along line Axe2x80x94A of the plan view, both showing a conventional two-dimensional image sensor. FIG. 5 is a cross-sectional view showing a single pixel in the two-dimensional image sensor of FIG. 4. The cross-sectional view of FIG. 4 does not partly show the structure in detail so much as FIG. 5.
The two-dimensional image sensor of FIG. 4 and FIG. 5 has a basic structure in which a photoconductive film 102 and a common electrode (upper common electrode) 103 are formed in this order on the active matrix substrate 101.
The active matrix substrate 101 includes: a glass substrate 104; TFTs (Thin Film Transistors) 105 as switching elements and charge storage capacitances (Cs capacitances) 106 provided on the glass substrate 104; and pixel electrodes 107 provided so as to cover all these members. Pixels, each of which is constituted by the TFT 105, the charge storage capacitance 106, and the pixel electrode 107, are arranged in an X-Y matrix form (two-dimensional matrix form).
The TFT 105 is constituted by a gate electrode 108, a gate insulating film 109, an a-Si (amorphous silicon) layer (i-layer) 110, an a-Si layer (n+ layer) 111, a source electrode 112, and a drain electrode 113. The charge storage capacitance 106 is constituted by a storage capacitance electrode (Cs electrode) 114, a gate insulating film 109, a storage capacitance electrode 113 (which also acts as the drain electrode 113).
The pixel electrode 107 is electrically insulated from the TFT 105 and the charge storage capacitance 106, as well as electrode wires connected to these members, by an intervening insulating layer (insulating protection layer) 115 and an insulating layer (insulating protection layer) 116. The pixel electrode 107 and the drain electrode 113 are electrically coupled via a contact hole formed through the insulating layers 115 and 116.
The photoconductive film 102 is provided so as to cover the pixel electrodes 107 and the insulating layer 116 in the active matrix substrate 101. The common electrode 103 is provided so as to cover the photoconductive film 102.
The photoconductive film 102, when irradiated with X-rays, produces electric charge (electron-hole) in it. The photoconductive film 102 is made from a semiconductor material, such as a-Se, that is selected depending on the wavelength of the electromagnetic radiation to be detected.
The common electrode 103 and the storage capacitance electrode 114 are arranged so that an electric voltage can be applied across them.
In the active matrix substrate 101, under the insulating layers 115 and 116 is formed a wiring layer 120 which includes, among others, the gate (scan), source (signal), and Cs (capacitance) lines coupled respectively to the gate electrodes 108, the source electrodes 112, and storage capacitance electrodes 114 for the individual pixels. The cross-sectional view of FIG. 4 shows a gate line as the wiring layer 120.
The wires extend to edges of the glass substrate 104 which are outside a pixel region 118 (region in which the pixel electrodes 107 are disposed). The wires are connected to a scan control circuit, a signal processing circuit, or another external circuit (not shown) at the edges of the glass substrate 104 via a TCP (Tape Carrier Package).
Now, operating principles of the two-dimensional image sensor will be explained. The photoconductive film 102 internally produces electric charge (electron-hole) when the photoconductive film 102 is irradiated with X-rays while voltage is being applied to the common electrode 103 and the storage capacitance electrode 114. The produced electric charge moves toward either the positive or negative electrode depending on the polarity of the applied voltage and stored in the charge storage capacitance 106.
The electric charge stored in the charge storage capacitance 106 is dischargeable through the source electrode 112 when the TFT 105 conducts in response to an input signal to the gate electrode 108.
The pixels, since being arranged in an X-Y matrix form as described above, is capable of producing two-dimensional information of the image, by sequentially feeding the gate electrodes 108 with a signal and detecting the electric charge discharged through the source electrodes 112 for each source line.
Incidentally, the efficiency of photoelectric conversion by the photoconductive film 102 varies depending on the material composing the photoconductive film 102. Typically, to achieve a desirable or even better efficiency, the photoconductive film 102 needs to be thick. For example, an a-Se photoconductive film 102 is deposited at a thickness of about 0.5 mm to 1.5 mm. In this case, the applied voltage should be as high as a few kilovolts.
Under these circumstances, if X-rays are shone excessively or the TFT 105 is turned off for an extended period of time, as electric charge builds up in the detecting of the image, a high voltage, which is almost equal to the applied voltage at maximum, is applied across the pixel electrode 107. Therefore, the applied voltage of a few kilovolts may place a voltage load as high as a few kilovolts at maximum to the pixel electrodes 107.
When this voltage load becomes greater than the tolerable voltage of the TFT 105 or the charge storage capacitance 106, the TFT 105 or the charge storage capacitance 106 are destructed due to insulation breakdown, seriously affecting the operation of the image sensor.
Related to this problem, some measures have been devised to prevent the destruction of the TFT 105 and the charge storage capacitance 106. Specifically, Japanese Laid-Open Patent Application No. 6-342098/1994 and other documents disclose protector capacitance; Japanese Laid-Open Patent Application No. 10-170658/1998 (Tokukaihei 10-170658; published on Jun. 26, 1998) and other documents disclose discharge of excessively stored electric charge by a protection circuit separately provided in the pixel; Brad Polischuk, et al., Direct Conversion Detector for Digital Mammography, SPIE Physics of Medical Imaging, 1999, Vol. 3659, pages 417-425 (published on May 1999) and other documents disclose discharge of excessively stored electric charge by means of voltage breakdown properties of the TFT element; and PCT WO96/34416 (published on Oct. 31, 1996) and other documents disclose discharge of excessively stored electric charge by means of a double gate structure TFT.
These measures, except those involving the formation of a protector capacitance, basically have limited effects only in the pixel region 118. The formation of the protector capacitance, depending on where it is formed, produces expanded effects beyond the pixel region 118, but is unsuitable to taking motion pictures because the resetting of the stored electric charge takes some time.
Now, we will consider problems related to the voltage load in a peripheral region R which is an area surrounding the pixel region 118.
An image can be detected by means of projection of X-rays in an image detecting region which is defined as the area in which the pixel region 118, the region in which the photoconductive film 102 is provided, and the region in which the common electrode 103 is provided are stacked in a normal direction to the surface of the glass substrate 104.
Therefore, if the image detecting region is to be provided utilizing the pixel region 118 as much as possible, the pixel region 118 needs to be covered entirely with the photoconductive film 102 and the common electrode 103. Typically, positioning accuracy is taken into consideration in design of the image detecting region as shown in FIG. 4, and the photoconductive film 102 and the common electrode 103 are provided extending beyond the borders of the pixel region 118.
Besides, in the peripheral region R surrounding the pixel region 118 on the active matrix substrate 101, the wiring layer 120 in which scan lines, signal lines, and capacitance lines are provided is located under the insulating layers 115 and 116.
Therefore, in the peripheral region R, in the region where the wiring layer 120, the photoconductive film 102, and the common electrode 103 are stacked, a high voltage may possibly be applied to the insulating layers 115 and 116 on the wiring layer 120 due to electric charge produced in the photoconductive film 102 as a result of projection of excessive X-rays. This renders it possible for the insulating layers 115 and 116 to suffer insulation breakdown similarly to the pixel region 118.
Thus, the charge storage capacitance 106 and the TFT 105, as well as the scan control circuit and the signal processing circuit, may be possibly destructed, for they are all connected to the wires in the wiring layer 120.
Even with no excessive X-ray projection, if a high DC voltage is applied for an extended period of time to the common electrode 103 provided over the wiring layer 120 in the peripheral region R, the electric charge produced in the photoconductive film 102 is gradually attracted toward the insulating layers 115 and 116. Thus, electric charge is stored on the insulating layers 115 and 116, seriously affecting signals in the scan lines, the signal lines, and the capacitance lines in some instances.
In view of these problems, the present invention has an object to prevent a high voltage application resulting from excessive projection of X-rays outside a pixel region from causing an insulation breakdown of a two-dimensional image sensor incorporating a photoconductive semiconductor and other layers provided on an active matrix substrate. Hence, a physically highly reliable, two-dimensional image sensor can be presented which can detect both still images and motion pictures.
In order to accomplish the above object, a two-dimensional image sensor in accordance with the present invention includes:
a conversion layer for converting electromagnetic radiation carrying image information to electric charge;
a pixel substrate including: a pixel accommodating layer for accommodating pixel electrodes connected to the conversion layer to accumulate the electric charge in the conversion layer; a wiring layer, located opposite the conversion layer across the pixel accommodating layer, for providing electrode wires to detect the accumulated electric charge; and an insulating layer interposed between the pixel accommodating layer and the wiring layer; and
an upper electrode, located opposite the pixel accommodating layer across the conversion layer, for developing an electric field between itself and the pixel electrodes,
wherein:
the upper electrode is provided in a pixel region in which the pixel electrodes are disposed.
According to the arrangement, the pixel substrate is constituted by a wiring layer for providing electrode wires for detecting electric charge transmitted from pixel electrodes, an insulating layer, and a pixel accommodating layer for accommodating the pixel electrodes, the layers being stacked in this order. On the pixel accommodating layer of the pixel substrate are there provided a conversion layer for converting electromagnetic radiation to electric charge and an upper electrode for developing an electric field between itself and the pixel electrodes in this order to form a two-dimensional image sensor. Also, according to the arrangement, the upper electrode is provided in a pixel region in which the pixel electrodes are disposed.
According to the arrangement, the upper electrode is provided in the pixel region; therefore, no upper electrode is provided above the conversion layer outside the pixel region. Consequently, little voltage is applied to the portion of the conversion layer outside the pixel region. As a result, even when an electric field develops between the upper electrode and the pixel electrodes, an electric field hardly develops in the direction from the upper electrode to the electrode wires provided under the insulating layer.
This can restrain the electric charge produced in the conversion layer from being stored on the insulating layer above the electrode wires due to effects of the electric field. Therefore, the electric charge can be prevented from excessively stored on the insulating layer, and thus the insulating layer can be protected from insulation breakdown.
Further, since the electric charge is restrained from being stored on the insulating layer above the electrode wires, it becomes possible to restrain increases in the potential of the electrode wires due to the stored electric charge. This can restrain variations in the potential of the electrode wires and thereby retrain noise in signals transmitted through the electrode wires.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.